I. Field of the Invention
The present invention generally relates to wireless communications networks. More particularly, the present invention relates to a method and apparatus for synchronizing timing in circuits associated with wireless communication terminals that support multiple airlinks or multi-mode phones.
II. Related Art
There are presently many different types of radiotelephone or wireless communication systems, including different terrestrial based wireless communication systems and different satellite based wireless communication systems. The different terrestrial based wireless systems can include Personal Communications Service (PCS) and cellular systems. Examples of known cellular systems include the cellular Analog Advanced Mobile Phone System (AMPS), and the following digital cellular systems: Code Division Multiple Access (CDMA) systems; Time Division Multiple Access (TDMA) systems; and newer hybrid digital communication systems using both TDMA and CDMA technologies.
The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSpread Spectrum Multiple Access Communication System Using Satellite Or Terrestrial Repeatersxe2x80x9d and U.S. Pat. No. 5,103,459, entitled xe2x80x9cSystem And Method For Generating Signal Waveforms In A CDMA Cellular Telephone System,xe2x80x9d both of which are assigned to the assignee of the present invention and are incorporated herein by reference.
The method for providing CDMA mobile communications was standardized in the United States by the Telecommunications Industry Association in TIA/EIA/IS-95-A entitled xe2x80x9cMobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,xe2x80x9d referred to herein as IS-95. Combined AMPS and CDMA systems are described in TIA/EIA Standard IS-98. Other communications systems are described in the IMT-2000/UM, or International Mobile Telecommunications System 2000/Universal Mobile Telecommunications System, standards covering what are referred to as wideband CDMA (WCDMA), cdma2000 (such as cdma2000 1xc3x97 or 3xc3x97 standards, for example) or TD-SCDMA.
In the above patents, CDMA techniques are disclosed in which a large number of mobile station users, each having a transceiver, communicate through satellite repeaters or terrestrial base stations. The satellite repeaters are known as gateways and the terrestrial base stations are known as cell base stations or cell-sites. The gateways provide communication links for connecting a user terminal to other user terminals or users of other communications systems, such as a public telephone switching network. By using CDMA communications, the frequency spectrum can be used by multiple terminals thus permitting an increase in system user capacity. The use of CDMA techniques result in much higher spectral efficiency than can be achieved using other multiple access techniques.
In a typical CDMA communications systems, both the remote units and the base stations discriminate the simultaneously received signals from one another using modulation and demodulation of the transmitted data with high frequency pseudo-noise (PN) codes, orthogonal Walsh codes, or both. For example, in the forward link, i.e., base station to mobile station direction, IS-95 separates transmissions for different users from the same base station into different channels by the use of different Walsh codes for each transmission, while the transmissions from different base stations are distinguished by the use of a uniquely offset PN code. In the reverse link, i.e., mobile station to base station direction, different PN sequences are used to distinguish different channels or user terminals.
The forward CDMA link includes a pilot channel, a synchronization (sync)-channel, one or more paging channels, and a larger number of traffic channels. The reverse link includes an access channel and a number of traffic channels. The pilot channel transmits a beacon signal, known as a pilot signal, and is used to alert mobile stations of the presence of a CDMA compliant base station. After a mobile station has successfully acquired the pilot signal, it can then receive and demodulate the sync-channel in order to achieve frame level synchronization and system time, etc. The synch channel carries a repeating message that specifically identifies the base station, provides system level timing, and provides the absolute phase of the pilot signal. This feature will be discussed in greater detail below. The paging channel is used by the base station to assign communication channels and to communicate with the mobile station when it has not been assigned to a traffic channel. Finally, the traffic channels assigned to individual mobile stations are used to carry user communications traffic, such as speech and data.
To communicate properly in a CDMA system, the state of the particular codes selected must be synchronized at the base station and mobile station. Code level synchronization is achieved when the state of the codes at the mobile station system are the same as those in the base station, less some offset to account for processing and transmission delay. In IS-95, such synchronization is facilitated by the transmission of the pilot signal, which comprises the repeated transmission of the uniquely offset PN code (pilot PN code), from each base station. In addition to facilitating synchronization at the Pilot PN code level, the pilot channel allows identification of each base station relative to the other base stations located around it using the pilot channel phase offset. The pilot channel, therefore, provides the mobile station with access to a first level of detailed PN sequence timing information.
Mobile stations initially acquire an IS-95 based communications system by searching for a valid pilot signal within a definable search window. Pilot signals associated with different base stations are distinguished from one another on the basis of the phase (time offset) of the pilot signal. Thus, although each base station transmits an identical pilot signal, pilot signals from different base stations have different phases. A 9-bit number can be used to identify the pilot phase and is called the pilot offset.
After a mobile phone has acquired a valid pilot signal and has associated that pilot signal with a particular base station, the mobile station can receive and demodulate the sync channel. In addition to providing the mobile station with the phase of the pilot signal and identification of its associated base station, the synchronization message also includes CDMA system level timing information. Although system time can be provided through a number of different timing sources, traditional wireless communication systems derive system timing information through the global positioning system (GPS) satellite system located at each base station.
Due in part to convenience and availability of mobile phones, in the United States the Federal Communications Commission (FCC) now requires that wireless communication system (WCS) providers implement a mechanism to automatically route 911 calls to the nearest emergency services processing center. This is referred to as the E911 requirement. In order to accommodate this requirement, the WCS must be able to quickly and accurately determine the geographic position of a mobile phone or wireless device. Conventional WCSs typically determine a user""s or mobile station position using what is referred to as either a handset based solution or a network based solution.
The conventionally used handset-based method typically relies on GPS capabilities to provide user position information. This GPS solution, however, exhibits degraded performance and availability in areas where satellite coverage is limited or obscured, such as indoors or in major urban areas. GPS solutions are also relatively slow to provide a position determination and can be costly. Network-based solutions rely on a signal transmitted from the mobile station to multiple fixed base stations. The limitation here, however, is the requirement of multiple base stations. Thus, if a user is in an area with limited base station coverage, providing position information using the network-based solution will be problematic.
Another issue to consider regarding GPS based solutions is that a GPS acquisition device may have a substantial number of search spaces, involving different codes, timing and so forth, to search over. The size of the timing window for the various timing hypothesis needing to be tested to acquire the GPS signal may be significantly large. This requires more time than desired to acquire the signal timing or to perform GPS measurements, and also affects the accuracy of the measurements.
What is needed, therefore, is a system and method to eliminate the shortcomings of the conventionally used position determination techniques. In particular, what is needed is a system and method of implementing a multi-mode dual circuit or ASIC wireless device, that can share accurate time among multiple ASICs. The sharing of system time among signal processing circuits speeds the process of position determination and facilitates the broadcast of system time over the entire WCS network. Also, what is needed is a multi-mode phone constructed and arranged to facilitate the sharing of WCS system time between multiple ASICs within the same mobile phone. A system and method constructed and arranged in this manner can accommodate aspects of both the GPS solution and the network based solution discussed above and provides timely and accurate position data for support of the E911 feature and/or timely intersystem handover, such as CDMA to wideband CDMA (W-CDMA).
Exemplary embodiments comprise apparatus and methods using a first communications device having at least first and second types of communication paths which is adapted to receive first and second timing signals over the first type communication path and transmit data on the second type communication path. The data is transmitted in association with the received first timing signal. In some embodiments, the first type communication path comprises a forward link and the second type communication path includes a reverse link. The first and second timing signals can comprise pilot and synchronization channel message signals, respectively.
A processor is coupled to the first communications device and is configured to receive a second timing signal and produce a timing word. The timing word can comprise information related for example to adjustments for compensating for path delays. A second communications device having at least the first type of communication path is coupled to the processor and adapted to receive the timing word and the transmitted data and to derive synchronization information. The resulting derived synchronization information is related to the first timing signal, and the processor performs one or more operations in accordance with the received second timing signal and the derived synchronization information. The operations being performed comprise deriving timing information associated with the second communications device. Alternatively, the operations comprise acquiring one or more communications signals in accordance with the received second timing signal and the derived synchronization information. In some embodiments, the one or more communications signals comprise at least a timing signal associated with a positioning satellite network.
In exemplary embodiments, the first and second communications devices comprise first and second application specific integrated circuits.
The apparatus can further comprise a tuner coupled to the processor to receive a number of radio frequency signals based upon a first processor control signal and to produce a selected frequency signal as an output. A first switch coupled to the processor receives a second processor control signal, the selected frequency signal from the tuner, and selectively switches the selected frequency signal between two or more output ports.
In this embodiment, a first circuit is coupled to the processor, the tuner, and the first switch and comprises a first receive path, which can be a forward communications link, coupled to a first switch output port to receive the selected frequency signal. The selected frequency signal comprises a first type of timing component and a synchronization word which is output to the processor which produces a second type timing component in response. The timing component can comprise for example pseudorandom noise level timing information. A time tracking device is configured to receive the first and second timing components, to perform a first synchronization operation in response, and to produce a synchronization point in accordance with the first synchronization operation. A code generator coupled to the processor receives a communications message there from, and generates a data sequence and provides it to the time tracking device which produces a correlated data sequence based upon the synchronization point and the communications message. A first transmit path, which can be a reverse communications link, is configured to receive the correlated data sequence and produce a synchronized data stream and to transmit the synchronized data stream using a first transmit path output port.
A second switch is electrically coupled to the processor and to the first transmit path output port, has two or more second switch output ports and is configured to receive a third processor control signal and the synchronized data stream, and to selectively switch the received synchronized data stream between the two or more second switch output ports. A third switch comprising two third switch input ports and an output port, has a first input port coupled to the second of first switch output ports, and a second switch input port coupled to one of the second switch output ports. The third switch is configured to receive a fourth processor control signal, the selected frequency signal using the first third switch input port, and the synchronized data stream using the second third switch input port.
In this embodimdent, a second circuit is also coupled to the processor and the third switch comprises a second receive path, which can be a forward communications link, having an input coupled to the third switch output port to receive a selected one of the synchronized data stream and the selected frequency signal, and a correlator coupled to an output of the second receive path configured to detect the presence of the synchronized data stream when received and provide the synchronized data stream as an output. A time measurement mechanism has an input coupled to an output of the correlator and another input coupled to the processor, to receive the synchronized data stream from the correlator and the synchronization word from the processor, and to derive a timing window there from.
The method for acquiring a timing signal comprises receiving first and second timing signals, such as a pilot signal and a synchronization channel message, over the first type communications path of a first communications device, transmitting data over the second type communications path of the first communications device, the data being transmitted in association with the received first timing signal. The first type communication path can for example be a forward communications link and the second type communication path can be a reverse communications link. The second timing signal is received in the processor and converted into a timing word. The transmitted data, derived synchronization information, and the timing word are received in the second communications device, and one or more operations are performed in accordance with the received transmitted data and the timing word.
A further embodiment comprises a communication device configured to operate in first and second air link modes, such as those of satellite, CDMA, W-CDMA, and GSM based wireless communication systems, with at least first and second type communication paths adapted for facilitating communication at least during of the first air link mod. The communication device is configured to receive first and second timing signals on the first type communication path, first and second timing signal data respectively representative of the first and second timing signals being stored in a communication device memory, and to transmit communication data on the second type communication path, the communication data being transmitted in accordance with the first timing signal data The second air link mode includes at least a first type communication path configured for facilitating communication with the communication device during operation of the second air link mode, and is configured to receive the transmitted communication data and produce first type synchronization information there from. A timing mechanism is configured to activate during the second air link mode and to receive the first type synchronization information, and a processor coupled to the communication device is configured to receive the second timing signal data stored in the memory and produce second type synchronization information there from, which is provided to the timing mechanism during activation.
Features and advantages of the present invention include the ability to provide a multi-mode dual-chip phone for performing rapid GPS measurements in support of E911. The sharing of WCS system time can not only be accommodated between multiple processing circuits or ASICs but can also be accommodated within a single reconfigurable multi-mode ASIC. The sharing of WCS system time between multiple ASICs and in single re-configurable ASICs enables acquisition of GPS as well as other types of services more quickly and accurately. Finally, sharing of WCS system time minimizes the frequency of call drops due to the mobile phone performing GPS measurements.